A spark plug, by being mounted in a combustion device such as an internal combustion engine, is used for igniting a mixture or the like. In general, the spark plug includes an insulator having an axial hole extending in a direction of the axis, a center electrode inserted in a leading end side portion of the axial hole, and a metal shell provided on the outer periphery of the insulator. Also, a thread portion for threadedly engaging with a mounting hole of the combustion device, and a flanged seat portion, formed closer to a rear end side than the thread portion, which protrudes radially outward, are formed on the outer peripheral surface of the metal shell.
Furthermore, a technology is known wherein, in order to ensure airtightness in a combustion chamber, a ring-shaped gasket is provided around the neck of the thread portion, and the gasket is brought into contact with a bearing surface of the combustion device when the spark plug is mounted in the combustion device (for example, refer to JP-A-2008-108478). In addition, a technique wherein a leading end face of the seat portion is formed into a tapered surface tapering toward a leading end side in the direction of the axis, rather than providing the gasket, and the tapered surface is brought into direct contact with the bearing surface, is proposed for the standpoint of realizing more superior airtightness. With a configuration of bringing the tapered surface into contact with the bearing surface in this way, the tapered surface is formed in a flat and smooth shape with extremely few irregularities, and a wide range on the tapered surface is brought into contact with the bearing surface, thereby promising a further improvement in airtightness (for example, refer to Japanese Patent No. 4,092,826).
However, with the configuration of forming the tapered surface in a flat and smooth shape, as heretofore described, there is fear that airtightness decreases drastically simply by minute marks being formed on the tapered surface during a manufacturing process, a conveyance of the spark plug, or the like. Consequently, it is necessary to carry out a management of the tapered surface with extreme caution during the manufacturing process, conveyance, or the like, thus resulting in inferior productivity and manageability.